Fuel injectors having two concentric needle valves are known. The dual needle valves are typically used to inject one or two different fuel streams into a combustion cylinder of the engine. For direct injection engines, each power cylinder of the engine has a fuel injector capable of injecting one or more streams of fluid directly into the cylinder. For example, an engine capable of operating under different conditions may receive two different types of fuel or, alternatively, a single fuel but at different pressures and/or dispersion patterns.
U.S. Pat. No. 6,601,566 (the '566 patent), which issued on Aug. 5, 2003, and is assigned to Caterpillar Inc. of Peoria, Ill., the contents of which are incorporated herein in their entirety by reference, discloses one example of a known fuel injector having dual needles. The '566 patent discloses a fuel injector capable of injecting two distinct quantities of liquid fuel into a combustion cylinder of a dual fuel engine. The fuel injector has dual concentric check valves operating to open separate sets of orifices. These check valves are directly controlled independently from each other, and are used for pilot and main injection events.
U.S. Pat. No. 6,769,635 (the '635 patent), which issued on Aug. 3, 2004, and is assigned to Caterpillar Inc. of Peoria, Ill., the contents of which are incorporated herein in their entirety by reference, discloses another example of a known fuel injector having dual needles. The '635 patent discloses a fuel injector having a homogenous charge nozzle outlet set and a conventional nozzle outlet set controlled, respectively, by first and second needle valve members.
Known fuel injectors having dual needles do not always effectively prevent fuel leakage past the outer needle valve. It can be appreciated that the high fuel pressures present at the needle valve(s), thermal expansion effects during operation of the fuel injectors, and/or the fine tolerances required for proper fit and sealing of the various components, present obstacles to the manufacturing and operation of such injectors.
More specifically, the sealing arrangement for the outer needle valve of the injector disclosed in the '566 patent includes contact between two conical surfaces, one formed on the tip of the outer needle valve, and one formed on the inner portion of a housing. Proper fit and contact between the two conical surfaces may be very difficult to achieve in a large-scale manufacturing operation. Similarly, the sealing arrangement for the outer needle valve of the injector disclosed in the '635 patent includes either a conical surface interface, similar to the '566 patent, or a stepped bore accommodating contact between the outer needle in two directions. These and other known sealing arrangements may be prone to leakage because of the issues stated above.